THE BACTERIOPHAGE AND THE BACTERIUM 95 



re-inoculated into fresh bouillon. Normal B. coli cultures develop. 

 Transfers to agar give two plaques for the first tube, none for the other 

 two. The culture yielding the two plaques is again re-inoculated. A nor- 

 mal culture develops. The bacteriophage has been eliminated. 



This strain of anti-Shiga bacteriophage possesses, therefore, 

 an extremely feeble virulence for the strain of B. coli under test. 



Experiment XXVII. To 10 cc. of bouillon is added 1 drop of a concen- 

 trated suspension of Shiga bacilli (this should give a slight turbidity equal 

 to about 50,000,000 bacilli per cubic centimeter) and 1 drop of an equally 

 concentrated suspension of B. coli. This double suspension is then inocu- 

 lated with 0.01 cc. of the anti-Shiga bacteriophage used in the above 

 experiment. 



After twenty-four hours there is a slight turbidity. A new passage into 

 a double Shiga-colon suspension is made. Perfect lysis takes place after 

 eleven hours. 



The lysed suspension is then introduced, in a quantity of 0.04 cc., into a 

 simple suspension of B. coli. Lysis is complete in seven hours. 



The ultramicrobes have developed at the expense of the Shiga 

 bacilli, and thus being maintained in the medium they have 

 gradually acquired a virulence for B. coli. 



In the intestinal tract a bacteriophage never finds itself in the 

 presence of but a single bacterial species. And this experiment 

 permits us to comprehend the process of acquisition in vivo of 

 virulence by a bacteriophage for a given bacterium. 



